There is a progressive increase in the general background level of mercury in our North American environment. Inorganic mercury in the environment can be converted to organic mercurials in the biological food chain and accumulate in foodstuffs, principally as methylmercury. Methylmercury in humans, unlike inorganic and some other organic mercurials, is widely distributed in body tissues, producing a different pattern of clinical findings than inorganic or elemental mercury. Methylmercury freely passes through the placental barrier. Little is known about the embryopathic effects of chronic mercury intoxication on the primate. The investigation is designed to uncover deleterious effects on the fetus at the level of human "subclinical" exposure. The purpose of this experiment is to determine the threshold level of chronic exposure of pregnant monkey to methylmercury which produces neurophysiologic deficits in the offspring. Other parameters in the primate include embryopathic dose range, pathogenesis of the animals, size of brain and viscera and molecular mechanism of injury. Rodents are used to determine the embryopathic mechanisms. Subsequent completion of the studies in primates will yield more information more directly transposable to man. Findings in the rodents will be tested in primates where subtle sensory behavioral and intellectual changes can be tested.